Since the 1800's fingerprint information has been collected from human fingers and hands by means of ink and paper. For the purposes of this invention, the term fingerprint refers to the skin surface friction ridge detail of a single digit, or part of the friction ridge detail of a digit, or any portion of the skin surface friction ridge up to and including the entire hand. In recent years, various electronic fingerprint scanning systems have been developed utilizing optical, capacitance, direct pressure, thermal, and longitudinal-wave methods. Methods based upon longitudinal waves, including ultrasound, have proven to be highly accurate, since longitudinal waves are unaffected by grease, dirt, paint, ink and other substances commonly found on a person's skin.
Use of ultrasound typically employs a piezoelectric transducer to send an ultrasonic energy wave, often referred to as a pulse, through a transmitting media. The pulse partially reflects back at each interface between media. The reflected portion of the pulse can be used to determine the distance traveled by the pulse, and this can be done for each partially reflecting interface. However, not all of the reflected pulses are of interest. For example, when a fingerprint is of interest, the pulse reflected by interfaces other than where the finger resides are not of interest. Since pulses reflected by the various interfaces will arrive at different times, it is possible to identify those pulses that are of interest by monitoring a time interval during which the reflected pulse for that interface is expected to arrive. This process is often referred to as range gating or biasing. The reflected pulse received during the expected time is then processed, often by converting it to digital values that represent signal strength. Through a single pixel sweep scanning device, information from a reflected pulse can be graphically displayed as a three-dimensional contour map of the object of a human finger, thumb or other skin surface. With respect to interface surfaces that are not flat, the depth of any gap structure detail (e.g. fingerprint valleys) can be displayed as a gray-scale bitmap image.
Although ultrasound imaging of a fingerprint is superior in detail to a similar image collected by an optical system or other means, we have discovered an arrangement of components which provides a superior image.